Just one poem, it seems, is all there is (8)
I believe the answer is:
universe
'is all there is' is the definition.
(I've seen this before)
'just one poem it' is the wordplay.
I cannot really see how this works, but
'one' could be 'un' (northern English dialect form of 'one') and 'un' is found in the answer.
'poem' could be 'verse' (verse is a kind of poem) and 'verse' is found within the answer.
A single letter 'i' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'seems' acts as a link.
Can you help me to learn more?
(Other definitions for universe that I've seen before include "The earth and all the heavenly bodies" , "Edible crustacean" , "sphere of activity" , "Sphere of thought" , "The whole of everything" .)